1. Field of the Invention
The present invention relates to a method and apparatus for recording information on a recording medium such as tape, disk or card and verifying the recorded information.
2. Related Background Art
In a prior art information recording apparatus, a verification operation to check whether correct recording has been done is commonly carried out. FIG. 1 shows an example of such a prior art information recording apparatus which verifies the recorded information.
In FIG. 1, numeral 1 denotes a drive controller, numeral 2 denotes a system controller, and numeral 9 denotes an error correction code (ECC) circuit. When a command to record information is sent from a host computer (not shown), the system controller 2 issues a command to the drive controller 1 to start the system. Data to be recorded, which has been sent from the host computer, has an ECC added thereto by the ECC circuit 9 and the resulting data is sent to an encoder 3. The data from the ECC circuit 9 is temporarily stored in a verification memory 10. The data supplied to the encoder 3 is sent to a recording transducer 4 as an encoded signal and recorded on a recording medium 5.
The signal recorded on the medium 5 is read by a reproducing transducer 6 and is sent to a decoder 8, which reproduces data from the signal. The reproduced data is compared with the data stored in the memory 10 by the system controller 2. When those data are equal, it is determined that the recording has been done correctly.
However, in the above prior art apparatus, because the read signal is supplied to the decoder for verification, depending on an error pattern and a characteristic of the decoder, an recording error may not be detected.
For example, let us consider a case where a 2-7 code proposed in U.S. Pat. No. 4,115,768 is used for encoding. FIG. 2 shows a conceptual view of a 2-7 code decoder. It is assumed that a correct signal "10001000" is input to a shift register 15 of the decoder as input data X. The signal is decoded by an arithmetic and logic circuit 16, a flip-flop 17 and a frequency divider 18 and a signal "1111" is produced as decoded data Y. On the other hand, if the input data is modified to "10101000" by an extra pulse generated by a defect on the medium, the decoded data is again "1111". The pattern "10101000" cannot exist in the 2-7 cord and is clearly a recording error. However, the signal produced by the decoder is same as that for the correct input signal, and the verification result shows no error. The recorded data which was misverified increases the number of errors as time elapses, and when it is to be reproduced later, the errors exceed an error correction ability, and reproduction may not be attained.
In the information recording apparatus described above, a synchronization pattern is usually added to the data to be recorded, and they are recorded on the medium. The synchronization pattern is used for synchronization during the reproduction of the data, and includes a PLL (phase locked loop) pull-in pattern for starting a PLL which generates a reference clock, a data mark (DM) for locating a start position of the data and a resynchronization pattern for correcting a shift of the clock. FIG. 3 shows an example of data having the PLL pull-in pattern and the DM pattern added thereto.
If an error is included in the synchronization pattern, the data cannot be reproduced. Accordingly, the synchronization pattern should be correctly recorded. However, in the prior art apparatus, the synchronization pattern is not directly verified but only whether the reproduction of the data by using the recorded synchronization pattern can be attained or not, is checked. Accordingly, even if the degree of errors in the synchronization pattern is low enough to permit the reproduction of the data at the time of verification, the errors may increase as time elapses and reproduction may not be attained later.